What license is needed for images used as part of a mobile vehicle wrap?

We are a non-profit community healthcare provider and operate a mobile dental van providing care to children in Nevada. We have an opportunity to rewrap the outside of the van and hope to use 20-24 images of children & families. We are unsure of the license we require as each image will only be used once; we see an average of 500 children on the van per year; and our entire patient base is 50,000 people. The van will only be used in the state of Nevada which has a population of 2.9 million and an area of 110,000 square miles.

Any advice we could receive on the correct license to use and/or ways to determine image views would be very appreciated!